Photographic processes using silver halide have been most widely used in the past due to their excellent photographic properties such as sensitivity or control of gradation, etc., as compared with other photographic processes, such as an electrophotographic process or a diazo photographic process. In the recent years, with respect to image formation processes for photographic materials using silver halide, many techniques capable of easily and rapidly obtaining images have been developed by changing the conventional wet process using a developing solution into a dye process such as a process using heat, etc.
Heat-developable photographic materials are known in the field of these techniques, and heat-developable photographic materials and processes have been described in U.S. Pat. Nos. 3,152,904, 3,301,678, 3,392,020 and 3,457,075, British Pat. Nos. 1,131,108 and 1,167,777, and Research Disclosure, No. 17029, pages 9 to 15 (June, 1978).
Many different processes for obtaining color images have been proposed. With respect to processes for forming color images by the reaction of an oxidation product of a developing agent with a coupler, it has been proposed to use a p-phenylenediamine type reducing agent and a phenolic coupler or an active methylene coupler as described in U.S. Pat. No. 3,531,286, a p-aminophenol type reducing agent as described in U.S. Pat. No. 3,761,270, a sulfonamidophenol type reducing agent as described in Belgian Pat. No. 802,519 and the combination of a sulfonamidophenol type reducing agent and a 4-equivalent coupler as described in U.S. Pat. No. 4,021,240.
Also, processes for forming a positive color image by a heat developable silver dye bleach process, with useful dyes and methods for bleaching have been described, for example, in Research Disclosure, No. 14433, pages 30 to 32 (April, 1976), ibid., No. 15227, pages 14 and 15 (Dec., 1976) and U.S. Pat. No. 4,235,957.
Moreover, a process for forming a color image utilizing a leuco dye has been described, for example, in U.S. Pat. Nos. 3,985,565 and 4,022,617.
However, these various processes for forming a color image by heat development are undesirable because discoloration and/or fading of the color image or coloration of the white background portion, etc., occurs because developed silver, the remaining silver salt and the remaining developing agent are coexistent after the heat development procedure. To eliminate this disadvantage, there has been proposed a process which comprises transferring the resulting color image to an image receiving sheet in the presence of a solvent such as methanol, etc., as described in U.S. Pat. Nos. 3,531,286, 3,761,270 and 4,021,240, etc. However, this process is not desirable from a standpoint of easy and rapid processing because of the increase in complicated steps.
With respect to processes for forming color images, the method using a compound capable of releasing a diffusible dye upon the coupling reaction with an oxidized product of an aromatic primary amine is described in Japanese Patent Application (OPI) Nos. 186744/82 and 207250/82.
Also, the method using a dye providing reducing compound capable of releasing a diffusible dye when oxidized is described in Japanese Patent Application (OPI) Nos. 179840/82 and 198458/82.
In these heat-developable color image forming methods, however, it is necessary to diffusion transfer the dyes released upon heat development into an image receiving element, in order to observe the color image formed. In a method for transferring the diffusible dye, which is described in these publications referred to above a surface of a photographic element or an image receiving element is wet with an organic solvent such as methanol, etc. or an oil and then both elements are superposed on each other whereby diffusion transfer is carried out. However, such a method is also undesirable in view of simplicity and quickness.
A method for omitting the dye diffusion transfer step involves incorporating a thermal solvent into the heat-developable color photographic material as described in Japanese Patent Application (OPI) Nos. 198458/82 and 179840/82. In these publications, there are described, as diffusion accelerators, some examples of non-hydrolizable organic compounds which are solid at room temperature (about 25.degree. C.) but become liquid at the time of heat development, that is thermal solvents. However, this method is disadvantageous since the coating property and film quality of the photographic material is degraded when such compounds are incorporated into the photographic material in an amount sufficient for conducting the diffusion transfer of dyes. Further, organic compounds which are liquid at room temperature exhibit a large effect on accelerating the diffusion transfer of dyes in comparison with organic compounds which are solid at room temperature, however, the former seriously degrade the coating property and film quality when they are incorporated into the photographic material.